There is no explicit requirement to put in anything.

The reason why the W-7 instructions say the W-7 should be placed on top of the return is that the IRS would know ITIN is missing on the return and that they will keep the return for later processing when the ITIN is issued.

The IRS already knows who is applying for the ITIN and which return it is linked to.  Indicating "Applied For" under SSN/ITIN adds no value.  Having said that, it doesn't hurt either if that's what you prefer.